1. Field of the Present Invention
The present invention relates to a driving device having an extremely compact size, and a light amount controller and a shutter using the driving device.
2. Description of the Related Art
Conventionally, motors that can be designed to be compact in size include a brushless motor. Brushless motors with simple driving circuits include a stepper motor, described below.
FIG. 9 shows an example of a small-sized hollow cylindrical stepper motor. The stepper motor includes two stators 102 each comprised of a bobbin 101 having a stator coil 105 concentrically wound thereon, two stator yokes 106 axially fixedly holding the bobbin 101 in a sandwiching fashion, the stator yokes 106 each having stator teeth 106a and 106b alternately arranged circumferentially about an inner peripheral surface of the bobbin 101, and a casing 103 to which are secured the stator yokes 106 formed integrally with the respective stator teeth 106a and 106b. One of the two casings 103 has a flange 115 and a bearing 108 fixed thereto, while the other casing 103 has another bearing 108 fixed thereto. A rotor 109 is comprised of a rotor magnet 111 rigidly fitted on a rotor shaft 110. The rotor magnet 111 defines radial gaps between the same and the stator yokes 106 of the stators 102. The rotor shaft 110 is rotatably supported by the two bearings 108 on opposite ends thereof.
A light controller using a variation of the stepper motor constructed as above has been proposed in Japanese Patent Publication (Kokoku) No. 53-2774. The light controller controls the quantity of passing light by opening and closing shutter blades connected to the stepper motor, in a stepwise fashion. Another variation is a hollow motor proposed in Japanese Laid-Open Patent Publication (Kokai) No. 57-166847. The hollow motor is a ring-shaped stepper motor which allows light to pass through a cavity formed in a central portion thereof.
In the above conventional small-sized stepper motor shown in FIG. 9, however, the casings 103, the bobbins 101, the stator coils 105 and the stator yokes 106 are concentrically arranged around the rotor 109, which inevitably results in increased outer dimensions of the motor. Further, magnetic flux generated by energization of the stator coils 105 mainly flows between the end faces 106a1 of the stator teeth 106a and the end faces 106b1 of the stator teeth 106b as shown in FIG. 10, and hence the magnetic flux does not effectively act on the rotor magnet 111.
Similarly, the light controller proposed in Japanese Patent Publication (Kokoku) No. 53-2774 and the hollow motor proposed in Japanese Laid-Open Patent Publication (Kokai) 57-166847 each have stator coils and stator yokes arranged around a rotor magnet, so that the outer dimensions of the motor are increased, and magnetic flux generated by energization of the stator coils does not effectively act on the rotor magnet.
Further, a coin-shaped brushless motor as shown in FIG. 11 has been proposed, e.g., in Japanese Laid-Open Patent Publications (Kokai) No. 7-213041 and No. 2000-50601. The brushless motor is comprised of a plurality of coils 301, 302, 303 and a disk-shaped magnet 304. As shown in FIG. 11, each of the coils has a thin coin-like shape, and is disposed such that the axis thereof extends parallel with that of the magnet. The disk-shaped magnet is magnetized in the axial direction thereof, and is disposed such that the magnetic surface (inner peripheral surface) of the magnet faces in a direction perpendicular to the axis of each of the coils.
In this motor, as illustrated by arrows in FIG. 12, magnetic flux generated by the coils cannot fully effectively act on the magnet. Further, the center of a rotational force or torque generated by the magnet is away from the outer periphery of the motor by a distance of L, so that the torque generated by the motor is small for the size of the motor. In addition, since a central portion of the motor is occupied by the coils and the magnet, it is difficult to utilize this portion for some other purposes.
It is a first object of the present invention to provide a small-sized and slim driving device which is easy to manufacture and capable of producing high output.
It is a second object of the present invention to provide a light amount controller using a small-sized and slim driving device which is easy to manufacture and capable of producing high output.
To attain the first object, in a first aspect of the present invention, there is provided a driving device comprising a magnet having a hollow cylindrical shape and having an outer peripheral surface and an inner peripheral surface, at least the outer peripheral surface being circumferentially divided into n sections magnetized so as to have alternately different poles, the magnet being rotatable about a rotational axis of the hollow cylindrical shape thereof, a coil arranged axially of the magnet and parallel thereto, an outer magnetic pole portion disposed to be magnetized by the coil, the outer magnetic pole portion having a plurality of hair comb tooth-shaped portions opposed to the magnet and arranged around the magnet such that an angle corresponding to a circumferential width of each of the hair comb tooth-shaped portions opposed to the magnet about the rotational axis of the hollow cylindrical shape of the magnet is equal to a predetermined angle A, and an inner magnetic pole portion disposed to be magnetized by the coil, the inner magnetic pole portion having a hollow cylindrical shape opposed to the inner peripheral surface of the magnet, wherein the predetermined angle A is set to such a value that a rotational force acts on the magnet to return to a position in which a boundary between adjacent ones of the n sections having alternately different poles is opposed to a circumferential center of a corresponding one of the hair comb tooth-shaped portions of the outer magnetic pole portion, when the boundary shifts from the position.
To attain the second object, in a second aspect of the present invention, there is provided a light amount controller comprising a magnet having a hollow cylindrical shape and having an outer peripheral surface and an inner peripheral surface, at least the outer peripheral surface being circumferentially divided into n sections magnetized so as to have alternately different poles, the magnet being rotatable about a rotational axis of the hollow cylindrical shape thereof, a coil arranged axially of the magnet and parallel thereto, an outer magnetic pole portion disposed to be magnetized by the coil, the outer magnetic pole portion having a plurality of hair comb tooth-shaped portions opposed to the magnet and arranged around the magnet such that an angle corresponding to a circumferential width of each of the hair comb tooth-shaped portions opposed to the magnet about the rotational axis of the hollow cylindrical shape of the magnet is equal to a predetermined angle A, and an inner magnetic pole portion disposed to be magnetized by the coil, the inner magnetic pole portion having a hollow cylindrical shape opposed to the inner peripheral surface of the magnet, and a light amount control member disposed for rotation in a manner interlocked to the magnet, wherein the predetermined angle A is set to such a value that a rotational force acts on the magnet to return to a position in which a boundary between adjacent ones of the n sections having alternately different poles is opposed to a circumferential center of a corresponding one of the hair comb tooth-shaped portions of the outer magnetic pole portion, when the boundary shifts from the position.
To attain the second object, in a third aspect of the present invention, there is provided a light amount controller comprising a magnet having a hollow cylindrical shape and having an outer peripheral surface and an inner peripheral surface, at least the outer peripheral surface being circumferentially divided into n sections magnetized so as to have alternately different poles, the magnet being rotatable about a rotational axis of the hollow cylindrical shape thereof, a coil arranged axially of the magnet and parallel thereto, an outer magnetic pole portion disposed to be magnetized by the coil, the outer magnetic pole portion having a plurality of hair comb tooth-shaped portions opposed to the magnet and arranged around the magnet such that an angle corresponding to a circumferential width of each of the hair comb tooth-shaped portions opposed to the magnet about the rotational axis of the hollow cylindrical shape of the magnet is equal to a predetermined angle A, and an inner magnetic pole portion disposed to be magnetized by the coil, the inner magnetic pole portion having a hollow cylindrical shape opposed to the inner peripheral surface of the magnet, a light amount control member being openable and closable in a manner interlocked to the magnet to thereby control an amount of light passing through the inner magnetic pole portion, and control means for controlling the light amount control member by selectively switching between a first state in which the magnet is held at a predetermined rotational position by attractive forces of the magnet and the outer magnetic pole portion when the coil is deenergized and a second state in which the magnet is rotated in a normal direction through a first predetermined angle from the first state by normal energization of the coil, and a third state in which the magnet is rotated in a direction opposite to the normal direction through a second predetermined angle from the first state by reverse energization of the coil, wherein the predetermined angle A is set to such a value that a rotational force acts on the magnet to return to a position in which a boundary between adjacent ones of the n sections having alternately different poles is opposed to a circumferential center of a corresponding one of the hair comb tooth-shaped portions of the outer magnetic pole portion, when the boundary shifts from the position.
In the first to third aspects of the present invention, preferably, assuming that the magnet has an outer diameter of D1 and an inner diameter of D2, the predetermined angle A is set to satisfy a condition defined by an expression:
A greater than (248.4/n)xe2x88x9258.86xc3x97(D1xe2x88x92D2)/(D1xc3x97xcfx80).
In the first to third aspects of the present invention, preferably, each of the hair comb tooth shaped portions of the outer magnetic pole portion has an axial length larger than a height of the outer peripheral surface of the magnet.
In the second and third aspects of the present invention, preferably, the light amount control member is openable and closable in a manner interlocked to the magnet to thereby control an amount of light passing through the inner magnetic pole portion.
According to the first to third aspects, the outer diameter of the driving device is determined by the outer magnetic pole portion opposed to the outer peripheral surface of the magnet, the inner diameter of the driving device is determined by the inner magnetic pole portion opposed to the inner peripheral surface of the magnet, and the axial height of the driving device is determined by the axial arrangement of the coil and the magnet. As a result, it is possible to largely reduce the size of the driving device. Further, the magnetic flux generated between the outer magnetic pole pieces and the inner magnetic pole portion magnetized by excitation of the coil crosses the magnet arranged between the magnetic pole pieces and member, and hence effectively acts on the magnet.
Furthermore, assuming that the angle corresponding to the circumferential width of each hair comb tooth-shaped portion of the outer magnetic pole portion opposed to the magnet about the rotational axis of the hollow cylindrical shape of the magnet is equal to the predetermined angle A, and the magnet has n magnetized poles, an outer diameter of D1 and an inner diameter of D2, the condition defined by the expression of A greater than (248.4/n)xe2x88x9258.86xc3x97(D1xe2x88x92D2)/(D1xc3x97xcfx80) holds. Therefore, when the coil is not energized, the center of each magnetized pole of the magnet can be stably held at a position opposed to the center of a corresponding hair comb tooth-shaped portion of the outer magnetic pole portion.
Moreover, since each hair comb tooth-shaped portion of the outer magnetic pole portion is configured to have an axial length larger than the height of the outer peripheral surface of the magnet, a force axially exerted on the magnet by the outer magnetic pole portion and the inner magnetic pole portion is reduced. As a result, sliding friction between the magnet and members holding the magnet in the axial direction is reduced, which enables smooth rotation of the magnet.
Further, the light amount controller according to the second and third aspects of the present invention includes the above driving device, and the light amount control member connected to the magnet of the driving device, for pivotal motion for control of the amount of light passing through the inside of the hollow cylindrical inner magnetic pole portion, and it is possible to allow light to pass through the central portion of the driving device.
Moreover, according to the third aspect of the present invention, it is possible to switch the light amount-controlled state between three different light amount-controlled states by changing the state and direction of energization of the coil.
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.